Thursday, 26 April 2012

Several editions of the Technic Civilization stories of Poul Anderson (Gregg Press, Ace Books, Baen Books) have attached to them a chronology compiled by Sandra Miesel, an excellent commentator on the works of Anderson. This chronology lists in internal chronological order all the stories and novels of the Technic Civilization series through periods like that of the Polesotechnic League and the Terran Empire. Miesel also added many annalistic notes to her chronology.

For those who wish to read the Technic Civilization stories in chronological order, or merely to have a list of the stories in a correct temporal sequence, Sandra Miesel has done readers of Anderson's works a real favor. However, commentators like Dr. Paul Shackley have discovered inconsistencies in some of Miesel's proposed dates which contradicts what the texts says.

For example, Miesel dates the birth of Nicholas van Rijn to AD 2376 and the crucial Polesotechnic League's Council of Hiawatha to 2400. However, as discussed by Dr. Shackley in his note "Inconsistencies II," Nicholas van Rijn was born too late to have attended that council. The section of Chapter IX of MIRKHEIM which discussed the Council of Hiawatha ended with "But when a century had passed--". Nicholas van Rijn could not have attended that council because he was 80 years old at the time of the Mirkheim/Baburite crisis. He would need to have been, implausibly, well over a century in age.

I have no objection to keeping Miesel's dating of the Council of Hiawatha to 2400, but I believe dating van Rijn's birth to 2424 is more accurate. And since the Mirkheim/Baburite war came when van Rijn was 80 years old, that means it should be dated 2504 (not in 2456, Miesel's date). This has the advantage of not contradicting what Chapter IX of MIRKHEIM said about "But when a century had passed."

Another error in Miesel's chronology contradicting what the texts say are her dates for "Lodestar" and MIRKHEIM. She dates the events in "Lodestar" and MIRKHEIM to 2446 and 2456. However, the Prologue to MIRKHEIM emphatically dates the events in that book to EIGHTEEN, not 10, years after "Lodestar." My revision of her chronology dates those stories to 2486 and 2504.

The next major inconsistency in Miesel's chronology contradicting what the texts say came from her dating the foundation of the Terran Empire to the 28th century and the birth of Dominic Flandry to AD 3000. These dates clash with what Chapter 10 of ENSIGN FLANDRY says, as the Merseian prime minister Brechdan Ironrede was going to the Imperial embassy for an official reception: "His destination was another offense, a compound of residences and offices in the garish bubble style of the Imperium four hundred years ago." This indicates the Empire had existed for over four centuries by the time of ENSIGN FLANDRY (because it is reasonable to think schools of architecture needed some time after the Empire arose to become popular).

However, one problem with this dating is that we don't know how long the year of the planet Merseia was when compared to our Earth's year. We don't know whether it was longer or shorter than the Terran year. Nonetheless it does indicates the Empire was older than the three centuries or so her chronology gives it by the time of ENSIGN FLANDRY.

Moreover, Miesel herself contradicts her chronology when she wrote in her "Introduction" for THE PEOPLE OF THE WIND (Gregg Press: 1977): "The Empire is its third century when it moves against the Domain in its first aggressive campaign against a civilized foe." Another chronological indication can be found in Chapter 8 of Anderson's THE DAY OF THEIR RETURN, as Ivar Frederiksen briefly summarized the history of relations between the Empire and the Domain of Ythri: "Still, it [the Domain] grew. So did Empire, Terra's, that is, till they met and clashed. Couple centuries ago, they fought." Now, if the Empire had existed a little over two centuries by the time of the Ythrian War and then that conflict was at least two centuries in the past by the time of THE DAY OF THEIR RETURN, that can only mean it had lasted more than four centuries by then.

Therefore, I would argue for dating the birth of Dominic Flandry to AD 3100, not 3000 (the year Miesel chose). The later date better fits the chronological evidence I collected from the texts. I am still puzzled how Miesel could have missed, for example, such crucial indications as the Prologue of MIRKHEIM saying the Baburite war occurred 18 years after "Lodestar."

In addition, I suggested below that Josip died in 3142 rather than in 3141 (Miesel's date was 3041) because a slightly longer reign for that Emperor fitted better the background of the stories. That is, it gives more time for the events recorded in those stories to take place without being crowded together too tightly.

If the argument I gave above is correct, then that means many, not all, of the dates given by Miesel in her chronology needs to be changed. Mostly by proposing dates later than the ones she chose. In the chronology given by me below, the dates I advocate are given first while Miesel's dates are given in square brackets. For the most part I used the "Chronology of Technic Civilization" to be found in the Gregg Press edition of ENSIGN FLANDRY. I also thought it best, for simplicity's sake, to omit many of the annalistic notes added by Sandra Miesel. I omitted many bibliographical details for similar reasons.

Tuesday, 24 April 2012

The concept of time travel raises at least three questions. Is it logically possible? Is it physically possible? How is it treated in fiction? I have read (a lot of) time travel fiction and studied philosophy, including logic, but not physics. I discuss the logic of time travel here and here but, in the current article, it is mentioned only to introduce the fiction.

"Travel" just to the future is time dilation or temporal stasis. Genuine time travel necessarily involves at least the possibility of pastwards travel, from the present to the past or from the future back to the present. Thus, Wells’ Time Traveler returns; his Sleeper doesn’t.

Time travelers may:

disappear at one time and appear at another;experience futurewards or pastwards time dilation;rotate around a massive gravitational field.

The first case is the easiest and simplest. The third is a contribution from later physical theories. In the second, Wellsian, case, it is almost always assumed that time travelers, while traveling, are undetectable by others. This assumption, despite Wells’ defense of it, is questionable but simplifies story telling. It is part of the "treatment in fiction," not of the "logic." The assumption enables a time traveler to arrive in the future without being expected or to arrive in the past without his journey from the future then becoming visible to others.

A pastwards time traveler’s arrival precedes his departure. Thus, an effect precedes its cause and might either cause or prevent it so the causality paradoxes are circular causality and causality violation. A "paradox" is either an apparent or a logical contradiction. The occurrence of apparent contradictions is unexpected but the occurrence of logical contradictions, or "contradictions in terms," is impossible. An event that occurs, because it has an effect, and does not occur, because it is prevented by its effect, is a logical contradiction – unless it occurs in one timeline and does not occur in another. Alternatively, within a single timeline, an uncaused event might prevent the event that would have caused it. Uncaused events, like causal circles, are counter-intuitive but not contradictory. Causal laws are empirical generalizations, not logical necessities.

Causality Violation

The idea of causality violation developed gradually.

(i) ImplicitThe earliest account of travel to the past that I know of, although I have not managed to read it, is "Missing One’s Coach," in which a modern man is mysteriously transported to a historical period. (1) Any account of travel to the past implicitly raises the question whether the time traveler would be able to affect (cause, change or prevent) past events.

(ii) Attempted?Mark Twain’s Connecticut Yankee, a modern man mysteriously transported to a, different, historical period, tries unsuccessfully to change past society without reflecting that success might have been paradoxical. (2) Perhaps he thought that his social revolution, even if successful in Arthurian Britain, would have been unrecorded and forgotten by the nineteenth century?

(iii) ImpliedIn The Time Machine by H. G. Wells, the Medical Man suggests that a time traveler observing the Battle of Hastings might attract attention as an anachronism while the narrator comments that time traveling "suggests curious possibilities of anachronism and of utter confusion." (3) (4). Pastwards travel is "anachronism". Causality violation would be perhaps the most extreme form of "utter confusion" but Wells did not elaborate.

(iv) SuccessfulIn Lest Darkness Fall by L. Sprague de Camp, a modern man, transported by lightning to yet another historical period, tries successfully to change the course of history. He prevents the Dark Ages. His success is explained by comparing time to a branching tree but also by comparing world lines to a tough web which it is difficult but not impossible to distort. (5) The second comparison continues the Wellsian conceptualization of time as a fourth dimension.

(v) AccidentalIn Bring the Jubilee by Ward Moore, a time traveler observing the Battle of Gettysburg attracts attention, thus unintentionally changing the outcome of the battle and the subsequent course of history. (6)

(vi) PreventedIn "Time Patrol" by Poul Anderson, a man from our future travels to post-Roman Britain in order to change the course of history but is apprehended by the Time Patrol, a time traveling organization that prevents other time travelers from either accidentally or deliberately changing the past. Causality violation is explained by de Camp’s second comparison. (7)

In "Delenda Est" by Poul Anderson, two time travelers intervene in a Roman-Carthaginian battle, thus deliberately changing the outcome of the battle and the subsequent course of history. Time Patrollers returning from the Pleistocene to 1960 find that they are in an alternative timeline, study history to locate the moment of divergence and restore their preferred timeline by counter-intervening in the pivotal battle. (8)

In Year of the Ransom, Time Patrollers prevent extra-temporal intervention in a later battle. (9) In "Women and Horses and Power and War," they lure time criminals to a particular period with false intelligence about the historical significance of an ancient battle. (10)

In "Amazement of the World," a random fluctuation in space-time energy changes the outcome of a medieval battle and the subsequent course of history. (11) Correcting the course of the battle does not restore the preferred timeline because the hero of the battle is a "personal causal nexus" whose world-line intersects with so many others that small changes in his career disproportionately affect the outcome of the medieval church-state conflict. (12) His actions prevent the birth either of a powerful Emperor or of a powerful Pope. By eliminating this knight, the Patrol restores the familiar history in which a stalemate between theocracy and autocracy allows the development of freedom and science.The Time Patrol series presupposes a single discontinuous timeline or a single mutable timeline or multiple successive timelines, although I argue here that there is only a terminological difference between "single mutable" and "multiple successive." "One timeline changes into others" and "many timelines succeed each other" mean the same thing.

A discontinuous timeline entails, counter-intuitively, that some events described in the stories do not occur. Mutable/successive timelines entail that deleted timelines existed in the past of a second temporal axis, not that they never existed, as the characters believe. However:

we can appreciate fictitious characters’ adventures without sharing their beliefs;Anderson, more than any other time travel writer, avoids overt contradictions and conceals subtler ones;his accounts of historical periods and analyses of historical processes make this series, like Jack Finney’s two Time novels, a culminating point of time travel fiction even if its logical basis is, partly, questionable.

(vii) OtherAlthough Finney’s Time novels are primarily about the experience of being in the past, each also culminates in a dramatic causality violation, although I argue here that these excellent novels are marred by inconsistencies when they address this paradox. (13) I also argue here that other authors’ accounts of causality violation are unacceptably inconsistent and that they contribute nothing to the literary development from Twain and Wells via de Camp and Moore to Anderson and Finney.

Using Modern Knowledge in the Past

The question whether a time traveler can change the past is a corollary of the simpler question: would a time traveler be able to use modern knowledge, techniques or equipment to his advantage while in the past? Any major application of modern knowledge etc in the past would have to change it. Using machine guns in an important ancient battle would, almost certainly, change the outcome of the battle and the subsequent course of history. However, guns might be used in an unimportant and unrecorded battle without any longer term consequences. The Time Patrol might allow or even instigate this for some reason as it does with other anachronistic equipment in "The Sorrow of Odin the Goth." (14) Perhaps the Connecticut Yankee thought that his attempted social changes would be unrecorded and forgotten.

A time traveler arriving in an earlier period carries within himself his modern knowledge and technical skills. However, if his means of time travel do not allow him to transport equipment, then it will be difficult or impossible to duplicate modern equipment in an earlier, especially a pre-industrial, period. Hank Morgan, the Connecticut Yankee, arrives with only the clothes that he wears but, impossibly, produces an endless supply of cables, telephones and machine guns and a printing press. He single-handedly and effortlessly masterminds a mostly clandestine industrial revolution in Arthurian Britain. Twain either did not realize or, more probably, did not care that this was impossible, and even had most of it occur off-stage, whereas his successors have paid more attention to details and practical difficulties.

De Camp’s Martin Padway struggles to innovate, e.g., to persuade Romans to accept Arabic numerals, and still has not synthesized gunpowder by the end of the novel, yet it is he, not Morgan, that accomplishes enough to divert history. Anderson’s "The Man who Came Early" is a direct reply to Lest Darkness Fall because its hero, like Padway, is transported to the past by a flash of lightning but he comments that this "…happens only once in many thousands of times." (15) Further, he not only does not introduce modern techniques but dies because he is unskilled in Viking customs and techniques. Against this, a boy in Anderson’s "The Little Monster" uses modern knowledge, techniques and a knife to survive in the Pliocene. (16)

In "Time Patrol," Stane’s easily transported energy weapon and his future sociological knowledge win him a place at court in fifth century Britain but, in "Delenda Est," a Time Patroller asked to manufacture a temporal vehicle in a pre-industrial period "…didn’t have the tools to make the tools to make what was needed…" (17)

Using Modern Knowledge in the Future

Affecting the future does not raise the same logical problem as affecting, and possibly altering, the past. However, we usually expect future societies to be too powerful or complex for a surviving modern man to influence them. This is the case in "Welcome" and "Time Heals" by Poul Anderson. (18) (19) On the other hand, the Time Traveler fights Morlocks with matches and explains them with Darwinism.Bob Wilson in "By His Bootstraps" by Robert Heinlein is a literary successor not only of the Time Traveller but also of Morgan and Padway because he not only visits a future paradisal society but also uses modern knowledge and equipment to take control of it. (20) Martin Saunders in "Flight to Forever" by Poul Anderson uses the time travel technique to win a space battle. (21)

Circular Causality

"The Chronic Argonauts" by H. G. Wells introduced but understated the circular causality paradox in a contemporary British setting. (22) The central character moves into an empty and shunned house, then travels through time while remaining within the house. On arriving in the past, he is attacked as an intruder by the previous occupants of the house but defends himself and returns to his present. In the absence of any evidence of an intruder, two brothers are convicted of the murder of their father killed in self-defense by the time traveler. Consequently, the house is empty and shunned. "A Stitch in Time" and "The Chronoclasm" by John Wyndham present uncomplicated accounts of the same paradox, also in contemporary British settings. (23) (24)

"By His Bootstraps," The Door into Summer and "-All You Zombies-" by Robert Heinlein present more complicated accounts of circular causality in future American settings. (25) (26) In "…All You Zombies," a sex-changed time traveller is both of his own parents. In "By His Bootstraps," Bob Wilson returns to 1952 to persuade his younger self to visit the future only to be interrupted by his older self trying to prevent him. The nuisance phone caller who interrupts their three-sided argument and the older man who sent Wilson back to 1952 are both himself.

In Beyond the Barrier by Damon Knight, a couple depart to seek a time traveler stranded in the past a moment after seeing themselves return safely with him. Their child does not accompany them but is not left unattended. (27)

The Technicolor Time Machine by Harry Harrison, The Anubis Gates by Tim Powers and three novels by Poul Anderson (The Corridors of Time, There Will Be Time and The Dancer from Atlantis) place circular causality in historical settings. (28) (29) (30) (31) (32)

A circle, once closed, is complete whereas anything that has been changed once can be changed again. Therefore, circular causality stories do not have sequels whereas Finney wrote a sequel to Time and Again and "Time Patrol" became a series. All the works mentioned so far in this section could be set in a single, continuous timeline. Such a timeline can accommodate causal circles but not causality violations because the latter require either a single, discontinuous timeline or several timelines.

Anderson incorporated circular causality into his causality violation scenario. In "Brave to be a King," "The Only Game in Town," "The Sorrow of Odin the Goth" and "Star of the Sea," the Patrol finds that it must intervene in history not to preserve but to cause its own past. (33) (34) (35) Sometimes the Patrol prevents causality violations by closing causal circles. After killing Stane, they retrieve the time vehicle that he had stolen but leave its fuel chest to be buried with him because it was "the singular contents of an ancient British barrow," mentioned in a work of literature, that had alerted the Patrol to unauthorized extra-temporal activity in post-Roman Britain. (36)

In "Ivory and Apes and Peacocks," Time Patroller Manson Everard, seeking time criminals in ancient Tyre, uncovers, by detective work, an account of how the Patrol had apprehended the criminals decades earlier. He then leads the expedition to apprehend them. (37)

I do not know whether other authors learned the circular causality paradox from "The Chronic Argonauts." However, this idea, having appeared in Wells, almost certainly came to Anderson from Heinlein, if not also from others. The idea of time travelers on historical battlefields passed from Wells to Moore and Anderson. Attempted historical causality violation passed from Twain via de Camp to Anderson. Thus, the Time Patrol series incorporates and perfects three strands of time travel fiction, including both causality paradoxes.

(Since writing this article, I have read The Time Traveller's Wife by Audrey Niffeneger, a major circular causality novel that is not genre sf (see below) but a novel of relationships with time travel integral to the plot. (38) Any novel can reflect on the passage of time and on the distance traveled from childhood but only time travel fiction can return a character physically to his remembered past. The Time Traveller's Wife is a third culmination of time travel fiction, following Anderson’s Time Patrol and Finney’s Time novels.)

(Later: I have just caught up with Richard Matheson's Bid Time Return/Somewhere In Time, a romantic novel involving time travel. Matheson has some good moments of circular causality and of what it would be like to be in the past. His means of time travel (see the next section) is identical with Finney's but Matheson's novel is ambiguous as to whether the central character, whose account we read, had imagined his chronokinesis.)

Means of Time Travel

Wells coined the terms "time machine," "time traveler" and "time traveling," later shortened to "time travel." Twain had used the phrase "transposition of epochs." Later authors coined the terms "chrononaut" and "chronokinesis" but Wells’ terminology remains in common use. "The Chronic Argonauts," which became The Time Machine, introduced the idea of a temporal vehicle.

The Time Traveler sits on the Time Machine and Time Patrollers sit on their timecycles but the BBC character Dr Who travels inside the TARDIS and Martin Saunders in "Flight to Forever" by Poul Anderson travels inside his time projector. Simon Morley in Jack Finney’s Time novels, Jack Havig in Poul Anderson’s There Will Be Time and Henry DeTamble in Audrey Niffeneger’s The Time Traveller's Wife do not need vehicles. Henry’s uncontrolled chrono-displacements are genetically based, therefore affect only his body, leaving behind even his clothes. Henry’s daughter, a second generation time traveler, sometimes controls when she travels to.

Despite his unaccountable sensation of "headlong motion," the Time Traveler remains spatially stationary while, effectively, fast forwarding, then rewinding, everything else. There is, therefore, no basis for his claim that the Machine, while "traveling," is undetectable because of its speed. In fact, he began his exposition by arguing that material bodies do not move along the fourth dimension but merely extend in that direction. If they did move along "time," then we would have to ask: what do they move in relation to? and: surely the Time Traveler, by accelerating, would leave everything else behind? - whereas he is instead described as seeing objects around him while traveling and then as finding them waiting for him on arrival. Further, motion along the fourth dimension would take time, therefore this dimension would not be time. In any case, a bullet or propeller is invisible because of its speed but we cannot safely occupy the same space as it.

Wells contradicts himself. In "The New Accelerator," he describes a drug which makes the bodies and minds of its inventor and the narrator function "many thousand times" faster than usual. (39) Even when they stand still, their visual and mental processes accelerate. Thus, they see a glass falling so slowly that it appears to them to be suspended in mid-air. Then, when they walk through Folkestone, too swiftly to be seen by anyone else:

"A purple-faced gentleman was frozen in the midst of a violent struggle to refold his newspaper against the wind…that had no existence so far as our sensations went…To see all that multitude changed to a picture, smitten rigid, as it were, into the semblance of realistic wax, was impossibly wonderful." (40)

Next, the inventor:

"…hopes to find a Retarder…it should enable the patient to spread a few seconds over many hours of ordinary time…" (41)

(The Time Traveler spreads a few minutes of his experience over many millennia of history.)

"…and so to maintain an apathetic inaction, a glacier like absence of alacrity, amidst the most animated or irritating surroundings." (41)

For many millennia, the Time Traveler, by comparison with his surroundings, is inactive, glacierlike, "smitten rigid" into the semblance of wax, and therefore should not be described as "going…fast" and blurred into invisibility. (42) His Machine is a Retarder, not an Accelerator, although, of course, what he notices and describes is not his own inaction but the comparative speed of everything else:

"The slowest snail that ever crawled dashed by too fast for me." (42)

Despite this, the Psychologist, one of the Time Traveler’s dinner guests, discussing the recently departed model time machine, says:

"We cannot see it, nor can we appreciate this machine, any more than we can the spoke of a wheel spinning, or a bullet flying…If it is traveling through time fifty times or a hundred times faster than we are, if it gets through a minute while we get through a second, the impression it creates will of course be only one-fiftieth or one-hundredth of what it would make if it were not traveling in time…" (43)

and, to demonstrate this, he passes his hand through the space in which the model time machine had been.

However, his account is incoherent. First, he says that the machine is traveling through time faster than the dinner guests, then he speaks as if it alone is traveling. Secondly, his "while" cannot mean "during the same period of time as." If the machine "gets through" a minute and the guests "get through" a second, then, at the end of these two processes of "getting through," they are not at the same time. The Psychologist means something like, "a time traveler would get through an objective minute in a subjective second whereas we get through only an objective second in a subjective second" or "a time traveler experiences a second while, during the same period of time as, everyone else experiences a minute." Thus, the time traveler is "Retarded," not "Accelerated."

The Psychologist further argues that this stretching (dilation) of the traveler’s duration causes an attenuation of his substance (although it is the Time Traveler who introduces the term "attenuated" shortly afterwards). (44) The material substance that usually fills a second is supposedly stretched out over a longer period to such an extent that it becomes invisible and even intangible. Such attenuation is not usually regarded as a consequence of relativistic time dilation and, even if it were, it would not be the same thing as the "headlong motion" that the Time Traveler says he experiences. Later writers usually accept the undetectability of time travelers in transit but not Wells’ explanation of it.

The static four-dimensional continuum expounded by the Time Traveler is presented more consistently in The Quincunx of Time by James Blish. Blish’s character, Thor Wald, explains:

"…if time is a linear dimension, just like length, height, and width, then the entity that I see before me as Captain Robin Weinbaum, is only a second-by-second section through a much larger entity, one of whose extensions is invisible to me." (45)

Weinbaum and Wald receive messages from their future and thus gain some knowledge of future events but cannot experience those events any sooner than anyone else. One of these messages links Quincunx to Midsummer Century by Blish. (46) In this second short novel, John Martels’ personality, though not his body, is projected from 1985 to 25,000. Personalities are semi-stable electromagnetic fields. Martels’ field is projected accidentally and received by a preserved brain. He could return to 1985, although he opts not to, so his "time-projection" is genuine time travel. His experience is consistent with the Time Traveler’s theory that it is our "consciousness" or "mental existences," not our bodies, that move along time. Wald adds:

"And the consciousness of Robin Weinbaum is moving along that entity in that invisible direction." (45)

There are problems with this theory. First, it contradicts the Time Traveler’s practice, though not Wald’s. Secondly, the Time Traveler describes "mental existences" not, e. g., as "fields" but as immaterial and dimensionless which suggests that they are undetectable and even non-existent. (47) Thirdly, as before, motion "along the Time-Dimension with a uniform velocity" would take time so this dimension would not be time. (47) Fourthly, the theory suggests that, before or after the passage through human bodies of their minds, these bodies are mere automata although indistinguishable from conscious organisms.

This radical mind-body dualism leads to solipsism. Each of us has no way of knowing whether other organisms are really conscious. My neighbor’s mind, if he has one, may move along the fourth dimension more slowly or quickly than mine. Thus, it may not be present in his body when I am speaking to him. Indeed, it is unnecessary:

"He is not free in any way to change the shaping of the ultimate creature; all he can do is observe…" (45)

"…the shaping of the ultimate creature…" includes the movements of his own body and even of its vocal organs. I remember Blish suggesting in a conversation at a science fiction convention that minds might move along time at different rates but this is surely an absurd conclusion to arrive at. If the conclusion is valid, then I cannot be sure that Blish’s consciousness was present when it was suggested.

Although I have tried to distinguish the literature from the logic, Wells in particular presents both. His introductory conversation, imitated by C. S. Lewis in "The Dark Tower" (although Lewis argues against time travel and for time viewing) is in the literary-philosophical tradition of Plato’s dialogues. (48) We can only do justice to The Time Machine by simultaneously appreciating it as fiction and arguing with its philosophy.

Returning to Blish, one message from the future comes from a ship that is somehow traveling physically backwards in time, from 8873 to 8704, but the recipients of the message do not know enough about the context to understand this. (49) Blish had hoped to base a time travel novel on the "finite, spinning universe" theory that allows world-lines to curve backwards through space-time. Perhaps the "world-line cruiser" mentioned in The Quincunx of Time was an early intimation of this.

Wells beautifully describes the fast forwarding universe:

"…night followed day like the flapping of a dark wing…the palpitation of night and day merged into one continuous greyness…the jerking sun became a streak of fire, a brilliant arch…The landscape was misty and vague…I saw trees growing and changing like puffs of vapor…I saw huge buildings rise up faint and fair, and pass like dreams…I saw great and splendid architecture rising about me, more massive than any buildings of our own time, and yet, as it seemed, built of glimmer and mist." (50)

Anderson partially matches this with his character Jack Havig in There Will Be Time:

" ‘I’m in a shadow world while I time-travel. Lighting varies from zero to gray. If I’m crossing more than one day-and-night period, it flickers. Objects look dim, foggy, flat.’ " (51)

But Havig wills himself backwards or forwards in time without needing a vehicle. He senses resistance, as if swimming against a high tide. Thus, his experience is more like a form of motion than is the Time Traveler’s. However, a four dimensional observer, with sufficiently keen perception, would see not Havig’s three dimensional body moving along the fourth dimension but his elongated, and attenuated, world-line stretching along it. Even if the attenuated world-lines of Havig and his fellow time travelers are undetectable, they must be statically present throughout space-time.

Unlike the Time Traveler or Havig, Martin Saunders sees only featureless greyness through the porthole of his time projector while it fast forwards the rest of the universe.

A Time Patroller on a timecycle experiences an instantaneous jump to a different set of spatio-temporal co-ordinates (although where does the energy for this come from?). The TARDIS is also a space-time vehicle, not just a time machine, but it and its occupants exist for a short period in another realm, the "time vortex," between disappearing from one place and time and appearing at another. Heinlein’s Bob Wilson steps instantaneously through a disk of nothingness called a Time Gate that he projects from the future after stepping through it.

Jack Finney’s Simon Morley enters earlier decades by self-hypnosis while wearing period costume in unchanged locations like old buildings. Finney’s fictitious physicist, Danziger, theorizes that this is possible because, according to Einstein, all times are real and the past still exists. However, Einstein’s space-time is the rigid four-dimensional continuum already mentioned. Its earlier decades are not only years ago in time but also light years away in space, not somehow just around the corner or out of sight. Finney’s Time novels would have had a sounder theoretical basis if Danziger, like Blish’s Adolph Haertel (Wald’s predecessor) had been presented as superseding, not implementing, Einstein’s ideas. Morley’s self-hypnosis gets him into the researched past and back to the remembered present but cannot get him into the unknowable future. However, Finney’s sf is nostalgic, not anticipatory, so the future is of no interest to him.

In Bring the Jubilee by Ward Moore, The Door into Summer by Robert Heinlein, "Wildcat" by Poul Anderson, "The Little Monster" by Poul Anderson, "The Face in the Photo" by Jack Finney and "The City That Was The World" by James Blish, a machine sends a character to another time but does not accompany him. (52) (53) (54) In "A Stitch in Time" by John Wyndham, a machine in the present lifts a character from the past, then puts him back.

In "Singularities Make Me Nervous" by Larry Niven and The Avatar by Poul Anderson, space travelers become time travelers by rotating around massive gravitational fields but Niven’s field is a black hole whereas Anderson’s is generated by a large artifact called a T machine, named after its theoretical inventor, F. J. Tipler. (55) (56) In "Rotating Cylinders and the Possibility of Global Causality Violation" by Larry Niven, named after a Tipler paper, some disaster always prevents the completion of a T machine because its completion in a single, continuous timeline would lead to the logical contradiction of causality violation. (57) A way to destroy your enemies is to let them steal the secret of T machines, unless a nova kills you first.

Olaf Stapledon’s Last Men and the characters in "The Long Remembering" by Poul Anderson observe past events through the brains and nervous systems of previous generations. (58) (59) The Last Men sometimes influence past minds, for example by inspiring Stapledon to write what he thinks is fiction.In "Welcome" by Poul Anderson, Barlow moves forward five hundred years in a superenergy state. In "Time Heals" by Poul Anderson, Hart moves forward almost nine hundred years in a low-entropy field. By my original criterion, these cases are not time travel because they are not reversible but Anderson does not need reversible techniques in stories that are only about what it would be like to get into the future.

In The Corridors of Time by Poul Anderson, warring factions called Wardens and Rangers walk or drive along corridors that have been rotated onto the temporal axis. Before entering a corridor, time travelers leaving different periods are separated by temporal intervals. After entering the corridor, they are, theoretically, separated only by spatial distances. Some will be near enough to see each other. Others will pass when moving along the corridor. Therefore, they should encounter and interact with each other and with their later and older selves on their first attempt to use the corridor.

However, such an outcome would complicate the story uncontrollably, especially since the factions are at war. An instant pitched battle inside the corridor would ensure that most individuals did not survive their first attempt to travel along the corridor and therefore never did re-enter it as older versions of themselves. Usually, however, the characters use the corridors without meeting each other. The only explanation given is:

"Duration occurs there too, but on a different plane…" (60)

The twentieth century protagonist, Lockridge, standing in a corridor with a companion, thinks:

"At any moment, someone might enter through some other gate and spy them. (Just what did that mean, here in this time which ran outside of time? He’d think about it later.)" (61)

If he does think about it, he does not do so in the novel. When Lockridge, pursued by Rangers, enters one of the corridors, his pursuers arrive in the corridor not a short distance away from him but a few moments after him. Thus, the order of events in the time "on a different plane" in the corridor follows the order of events in the familiar time outside the corridor. Again, this is convenient for story telling purposes. Lockridge would have been apprehended if his pursuers had arrived simultaneously with him.

If, in Lockridge’s experience, his first journey along a particular corridor starts from the twentieth century and his second journey along the same corridor starts from the fourteenth, which of the journeys will an observer inside the corridor witness first? We must imagine not an observer moving steadily along the corridor from the past towards the future but instead a stationary observer able to perceive the entire length of the corridor simultaneously.

As in the Time Patrol series, Anderson is good at writing his way around potential complications so that the reader is rarely troubled by them. The question in the preceding paragraph occurred to me only when writing about The Corridors of Time. Another potential problem arises when we are told that:

"Emergence cannot be precise, because the human body has a finite width equivalent to a couple of months. That was why we had to hold hands coming through – so we would not be separated by weeks." (60)

The Einsteinian space-time equivalence is not one body width to two months or "thirty-five days per foot" but one hundred and eighty six thousand miles to one second. (62)

Thor Wald, explains:

"Assuming, to keep the figures simple, that Robin lives to be a century old, he would then be roughly a foot thick, two feet wide, five feet five inches in height, and five hundred and eighty-six trillion, five hundred and sixty-nine billion, six hundred million miles in duration." (45)

Thus, Anderson’s corridors that endure for six thousand years would have to extend for six thousand light years in space before they were rotated into time but, in this novel, Anderson avoids mentioning Einsteinian space-time. If space-time equivalence is valid, then the simplest theory is that one of our three space dimensions becomes a corridor’s time dimension. It should be possible to construct a spatial corridor that allows entry to any period of the internal history of one of the corridors. This is another complication that Anderson does not need for the story that he wants to tell so it is not mentioned.

Time Patrol timecycles resemble the Time Machine. Jack Havig’s time traveling resembles the Time Traveller’s. The equation of time with length in The Corridors of Time recalls the discussion at the beginning of The Time Machine. Time travel to historical periods is not Wellsian (such a passage was cut from The Time Machine) but does continue the tradition of Twain and de Camp.

Anderson presents more varied and more imaginative means of time travel than anyone else. In particular, the Time Patrol and There Will Be Time are opposites: instantaneous vehicular space-time travel with causality violation as against lifespan-consuming psychic time travel without causality violation. Some means are mere formulaic premises for short stories but the temporal corridors and Havig’s power are original ideas and major features of the novels in which they appear.

Limits on Time Travel

Limits prevent paradoxes. For example, if time travel is limited to the remote past, then time travelers cannot intervene in recent history. If time travel is possible only in interstellar spaceships, then an individual inventor cannot make a machine that will take him to yesterday and return him to today while remaining in his laboratory. Thus, again, he cannot easily intervene in recent events.

The imprecision of emergence from a time corridor is one such limit. A time traveler who sets out to change or prevent a known event will invariably arrive after the event, if he encounters no mishaps en route. Even traveling pastwards to warn a colleague of an impending danger may be impossible:

"Such as could be sent have doubtless been baffled by the uncertainty factor; they emerged too early or too late." (63)

Time travelers know of some events in their own futures but not others:

"Lockridge was certain to reach Brann…That fact was in the structure of the universe. "However, details were unknown. (Like the aftermath…Did he or did he not get back alive? The margin of error in a gate made it unfeasible to check that in advance.)" (64)

Anderson imagines limits, then invents ways around them. In "Flight to Forever," Saunders travels from 1973 to 2073, then finds that he would need infinite energy to travel back more than about seventy five years but returns to 1973 by moving forwards around the circle of time.

In "Wildcat," it is only possible to travel forwards in hundred year hops and backwards by about 100,000,000 years. Therefore, it seems to be impossible to get information from the near future. However, when Team A, visiting the twenty first century, finds that Earth was sterilized by a nuclear war a year after the twentieth century base date, that Team places this data where it will be found by Team B which visits 100,000,000 A. D. and returns home. Team A joins a Jurassic base a century after its establishment. It is known that life will be re-evolving in 100,000,000 A. D. and hoped that descendants of the Jurassic settlers built spaceships.

In There Will Be Time, a few individuals from different periods can time travel by an act of will. They need no spatial journey or temporal vehicle. There seems to be no limit to their time travel ability. However, there are practical limitations. They can carry only a few pounds, including clothes. They are immobilized if attached to a larger mass (seized by an opponent, handcuffed to a captor or chained to a wall). They cannot breathe while traveling. (The Time Traveler did not encounter this difficulty. Perhaps the Time Machine carried some air along with it.) When ice covers the Earth’s surface for extended periods, unwary time travelers cannot emerge and risk suffocation. When the Earth is covered with water, they can emerge but only to drown. Jack Havig’s group carries miniature oxygen tanks when necessary but not all time travelers are so well equipped or organized.

A time traveler in There Will Be Time soon loses count of days flickering past, especially over centuries or millennia and when storms or buildings conceal the dawns. Casting about to zero in on the target date consumes lifespan. Havig’s "chronolog," small and light enough to be carried through time, detects heavenly bodies even through an overcast and flashes a light at his destination. Another time travel group, planning their return journey from the day of the Crucifixion but lacking a chronolog, erects, in the twenty first century ruins of Jerusalem, a big billboard with the date corrected daily.

The dates of some important events are not known. Anyone seeking the Crucifixion aims for Passover, 33 A. D., whether or not this is accurate, but they first have to travel to Jerusalem by ordinary means which did not include air travel until the twentieth century.

Despite the limits to his power, it would seem to be easy for Havig to change the past. The simplest experiment would be this: the time traveler remains in a room for two minutes; if his older self has not appeared after one minute, he travel pastwards for one minute; if he has appeared, he does not travel pastwards. A simple way to intervene in public events would be if he set out to appear in front of a television news reader on the previous day. However, Havig’s first attempt at causality violation is not a carefully planned experiment but an expedition to prevent his father’s death. He is prevented from setting out first by an accidental injury, then by business responsibilities, then by family commitments.

Eventually, like other time travelers, he stops trying. When a captured enemy escapes, they do not waste their efforts in trying to prevent the escape, which has happened, but attempt an early recapture, which may occur in their immediate future. Therefore, most events are as they are either because no time traveller wants to change them or because, knowing that some accident, possibly a fatal injury, will prevent any attempted change, they do not try. In these cases, there need not be any accident waiting to happen to interfering time travelers. The fact that time travelers do not attempt to change an event sufficiently explains why the event is not changed.

If an event occurs, then we know either that no one has tried to prevent it or that someone has tried and failed, whether or not the someone in question is a time traveler. But it is difficult to imagine what could thwart the "simplest experiment" suggested above, especially if it were attempted simultaneously in different places by a team of hundreds of experimentally minded time travelers. Would they all have heart attacks or be struck by lightning or meteors immediately before starting to travel into the past? Must we choose between a logical contradiction and a statistical impossibility?

(Added on 1 April 2007: Thanks to Colin G. Mackay for pointing out that there is no such thing as a "statistical impossibility." My argument should be re-phrased: Must we choose between a logical contradiction and a statistical improbability?)

I can think of only two explanations: either there are no time travelers (this may be the world that we live in) or there are very few and none of them is inclined to conduct this kind of experiment. In There Will Be Time, the group called the Eyrie is intent on exercising power and will do nothing to risk losing its power. Its members are unscientific and even superstitious. Havig’s rival group is morally responsible, intent only on overthrowing the Eyrie and then on doing something useful with time travel (although I still think that a few experiments would be appropriate).

Although Havig cannot change known events, he can sometimes change their significance. This starts on the day of the Crucifixion. Havig goes there hoping to meet other time travelers. The Eyrie goes there to recruit. Thus, for Havig and the Eyrie, Crucifixion day is merely a convenient rendezvous point. On that day, one of Havig’s fellow recruits is called Boris. Later, having defected from the Eyrie and formed a rival group, Havig will recruit Boris to that group, then send him to Jerusalem to infiltrate the Eyrie. Thus, we later learn, Boris must have recognized Havig on the day of their recruitment to the Eyrie but feigned ignorance. Thus also, Havig cannot prevent the Eyrie from recruiting Boris but can cause it to recruit him in circumstances that favor Havig’s cause, not the Eyrie’s.

The Sachem of the Eyrie cannot prevent the Maurai Federation from dominating international politics for three or four centuries after the nuclear war but does found and guide an American-based nation that he hopes will supplant the Maurai and restore white supremacy. He expects to succeed because he periodically visits the future Eyrie and sees that it prospers. He does not know that Havig’s group will defeat the Eyrie and fake the evidence of its success.

When an Eyrie recruiter prevents a thirteenth century time traveler from trying to rescue the Savior en route to Golgotha, Anderson adds that a Roman soldier would have killed the would-be rescuer a second later. This addition is unnecessary. Eyrie intervention is sufficient to explain why the Passion was not interrupted. This would, of course, involve a causal circle: the Eyrie would be able to recruit on the day of the Crucifixion because, while recruiting there, they ensured that the event occurred as described in the Gospels.

Of course, the crucifixion victim whom the time traveler had tried to rescue might not have been the historical Jesus. The Eyrie recruiter asks:

"How do you know that person really was your Lord?" (65)

I took this question to be philosophical or metaphysical but it may be simply historical. There were a lot of men called Jesus and a lot of crucifixion victims.

What would it be like to visit the past?

Any serious account of travel to the past raises this question. L. Sprague de Camp in Lest Darkness Fall and Tim Powers in The Anubis Gates evoke the experiences of modern men having to survive in earlier periods. The single most sustained answer is given by Jack Finney who perfected nostalgic time travel focused on turn of the century New York. His characters immerse themselves in images and records of an earlier year, then experience that year directly. They walk along streets that they have seen in old photographs.

However, Poul Anderson vividly describes the sights, sounds and smells of many historical periods. When Everard enters the Pasargadae of Cyrus the Great, we hear the street cries. (66) When he enters "Tyre of the purple" in 950 B. C., we see the docks and "the costly colors." (67) When he enters Bactra in 209 B. C.:

"…the scene was eerily half-familiar. He had witnessed its like in a score of different lands, in as many different centuries. Each was unique, but a prehistorically ancient kinship vibrated in them all." (68)

When Everard travels on foot through medieval Sicily, four pages describe the countryside that he walks through. (69) When Havig enters Jerusalem in 33 A. D.,Anderson lists the smells of horses, men, smoke, bread, leeks, garlic, grease, animal droppings, musk, roses, lumber and leather. (70) His time travelers move not through abstract time but through real history. Time Patrollers are immersed in ancient civilizations, Roman legions, medieval battlefields and barbarian settlements in European forests. Two Patrollers on a mission to the first century Low Countries camp on a hill surrounded by woods:

"They could have sprung back to Amsterdam’s comforts, but it would have wasted lifespan, not in the shuttling but in the commuting to and from quarters there, the shucking and redonning of barbarian garb, perhaps most the changes back and forth of mind-set. Let them rather dwell in this archaic land, become intimate not only with its people but with its natural world. Nature - the wilderness, the mysteries of day and night, summer and winter, storm, stars, growth, death – pervaded it and the souls of the folk. You could not really understand them, feel with them, until you had yourself entered into the forest and let it enter into you." (71)

A Patrol ethnographer who has lived for twelve years in the first century remarks, when visiting his colleagues’ camp:

In some passages, Anderson follows the experiences not of the Patrollers but of their acquaintances in the past and thus writes pure historical fiction. When a Time Patroller, mistaken for Wodan, has a son in the fourth century:

"Dagobert stayed unrestful…folk said that was the blood of his father in him, and that he heard the wind at the edge of the world forever calling. When he came back from his trip south, he brought news that a Roman lord hight Constantine had finally put down his rivals and become master of the whole Empire." (74)

Chapters recounting only the experiences of permanent dwellers in the past often begin with the weather: "Winter brought rain…" (75); "Wind rushed bitter…" (76); "Suddenly springtime billowed over the land." (77)

The concluding two sections of "The Sorrow of Odin the Goth," and the opening three sections of the following story, "Star of the Sea", cover:

time travelers in 43 A. D.

science fiction

the death of a king in 374 A. D.

historical fiction

a divine betrothal

mythological writing

the siege of a first century Roman camp

historical fiction

a time traveler in 1986 A. D.

science fiction

One Time Patroller, in 1990, specifically rejects Finney’s nostalgic approach to time travel:"The Midwest of his boyhood, before he went off to war in 1942, was like a dream, a world forever lost, already one with Troy and Carthage and the innocence of the Inuit. He had learned better than to return." (78)

Anderson writes poetically not only about past times but also about variable reality. Everard in Amsterdam in 1986 knows that "…all the life that pulsed in the city…" is:

"…a spectral flickering, diffraction rings across abstract, unstable space-time, a manifold brightness that at any instant could not only cease to be but cease ever having been." (79)

(Only the last clause is problematic. Causality violation in an earlier century might prevent Amsterdam from coming into existence but how could causality violation allow the city to exist until 1986, then annihilate it? Nothing can exist until a particular time, then, at that time, cease having existed until that time. The deletion of a four-dimensional space-time continuum might be an observable event in a fifth dimension functioning as a second temporal dimension but it cannot be an event within that same continuum. Analogously, if a book is thrown into a furnace, then it does not make sense to ask at what point in the text the entire text is destroyed. It is all destroyed together.)

What would it be like for a modern man to be transported to the remote future?

The classic answer remains The Time Machine. Wells describes his future as colorfully as Anderson and Finney describe their pasts. The modern man need not be transported by time travel specifically. Three successors to The Time Machine are "Flight to Forever" by Poul Anderson (time travel), A World Out Of Time by Larry Niven (time dilation) and Midsummer Century by James Blish ("time-projection"). (80) All four works not only describe a future period but also summarize a future history.

The Time Traveler fast forwards the growth of civilization, deduces the devolution into Morlocks and Eloi and witnesses the extinction of life before returning to the nineteenth century. Martin Saunders in "Flight to Forever" stops often enough to learn the course of galactic history before passing through heat death and re-creation into the twentieth century of the next identical cosmic cycle. In A World Out Of Time, Jerome Corbell’s identity survives for three million years by freezing, memory transfer, time dilation, cold sleep and rejuvenation. He reconstructs most of what has occurred in the Solar System in his absence. In Midsummer Century, when Martels’ consciousness has been projected from 1985 to 25,000, he learns of four successive future civilizations and helps to build a fifth.

Literature and Genre

A Connecticut Yankee in King Arthur’s Court is an American fantasy about a modern entrepreneur mysteriously transported to a historical period. The Time Machine is a British science fiction novel about a modern scientist technologically traveling to a remote future. They were the first major works on time travel. Like the first science fiction novel, Frankenstein by Mary Shelley, they predate the literary ghetto of genre science fiction and are generally recognized as contributions to world literature. (81)

In passing from Twain and Wells to de Camp and Heinlein, we pass not only from the original ideas to their fuller development but also from literature to genre. The genre includes:

action-adventure fiction with extraterrestrial, futuristic or other exotic settings;conceptual speculation, for example about the technology and paradoxes of time travel;serious fiction, for example about the predicaments of travelers to the past or future, the literary successors of the Yankee and the Time Traveler.

To illustrate this third point:

"It was a strange thing to meet her at intervals of months which for Havig were hours or days. Each time, she was so dizzyingly grown. In awe he felt a sense of that measureless river which he could swim but on which she could only be carried from darkness to darkness." (82)

In that paragraph, Anderson expresses universal experience, "…carried from darkness to darkness", and Wellsian imagination, "…that measureless river which he could swim…"

The Time Machine, originally described as a "fantastic and imaginative romance," combines conceptual speculation with serious fiction while incorporating action-adventure in the fights with the Morlocks and the encounter with giant crabs. Much genre sf emphasizes action-adventure and/or speculation at the expense of characterization and even of competent writing but other genre writers are worthy successors of Shelley, Twain and Wells.

In particular, Anderson’s sf ranges from effective action-adventure fiction to serious thematic and speculative novels, sometimes featuring the same characters. Dominic Flandry stars in mere space operas with the colorful setting of the Terran Empire, then in more substantial novels about imperial decline. Another reader described Anderson to me as a popular writer with literary pretensions. I think that it is the other way around. Anderson was a serious writer who enjoyed action-adventure so much that he sometimes introduced it into stories that would have been better without it. Thus, The Boat of a Million Years has an unnecessary few pages when the small group of immortals practices a deception in order to rescue one of its members from a hospital. (83) This could have happened off stage, if it even had to happen.

By contrast, the Time Patrol police actions and battle scenes, when they do occur, are integral to the narrative and secondary to the descriptions of past eras and the predicaments of the characters. The Time Patrol places technological time travel in historical settings, thus synthesizing the legacies of The Time Machine and A Connecticut Yankee. Uniquely, the Time Patrol is not a single story or novel about an individual time traveler experiencing one of the causality paradoxes in a particular historical period but a series of stories and novels about an organization of time travelers experiencing both causality paradoxes in many historical periods. Because it is a long sf series set in many historical periods, the Time Patrol is a more effective synthesis of sf with historical fiction than is Asimov’s celebrated Foundation series which merely projects the Roman Empire onto the Galaxy.

By contrast, Anderson:

projects the Roman Empire less implausibly onto a smaller volume of interstellar space and a shorter period of time in one of his eight future histories;speculates more credibly about interstellar travel in later future histories;shows the real Roman Empire in The Golden Slave (historical fiction), The King Of Ys tetralogy (historical fantasy, with Karen Anderson) and the Time Patrol series (historical sf); (84) (85)depicts the Viking period in a historical trilogy and in several fantasies, including a retold Saga; (86) (87)depicts the fourteenth century in historical fiction, historical fantasy and sf; (88) (89) (90)also synthesizes sf with historical fiction in a long novel about an immortal man who lives from before Christ into an indefinite future, thus experiences but cannot revisit historical periods (his single volume could have been two, a fictitious history with a futuristic sequel); (91)depicts alternative histories in the Time Patrol series and in several short stories and fantasy novels; (92) (93) (95) (95)imagines an inn between the worlds where characters from different periods, histories and fictions meet, although the Time Patrol excludes itself even from this multiverse by disallowing the peaceful coexistence of alternative timelines. (96)

Jules Verne’s fiction is collectively called "Extraordinary Voyages." Anderson’s could be called "Past and Future Histories."

Heinlein linked his ingenious time travel fiction to his Future History when a major character of the History, Lazarus Long, revisited the period of his own childhood in the early twentieth century. (97) Unfortunately, Heinlein wasted this excellent premise by focusing on a sentimentalized sexual relationship with the added shock value of incest: Long with his, at that time unknowing, mother. Asimov linked his logically inconsistent causality violation novel, The End of Eternity, to his Galactic Empire series when the prevention of time travel allowed the development of the Empire. See here. (98) Anderson linked his ingenious circular causality novel, There Will Be Time, to his Maurai future history when his time traveler, Jack Havig, visited the Maurai period.

In the Time Patrol series, we read of the Temporal language which enables Patrollers from different periods to communicate without being understood by others and which has tenses for time travel. In There Will Be Time, time travel is discussed, briefly, in Latin: "Es tu peregrinator temporis?" (65)What Anderson presents is both more and better. His Time Patrol, originally four stories, collected and re-arranged to present a beginning, a middle and a culmination, was already a major contribution to time travel fiction. Then, over a decade later, further stories, usually longer and more complicated and making original contributions to the time travel concept, began to be added so that the series is now several times its original length with possibilities for further growth although, sadly, Anderson is no longer alive to continue it. It is unique in time travel fiction and superior among sf series. Whereas other writers develop particular aspects of the time travel concept – historical veracity, futuristic speculation, the technology or one of the paradoxes – Anderson alone addresses every aspect.

Defining statement: the Time Travel Archives would be uniform editions of good time travel stories and novels covering the basic concept, the causality paradoxes and divergent timelines but excluding stories or novels whose internal logic of time travel was regarded by the Editor as unacceptably inconsistent.

However, works by Wells, Anderson and Finney that compensate for questionable logic in other ways would be included. A rigorous application of the defining statement would rule out The Time Machine, The Time Patrol and Time and Again.

The Basic Concept, Causality Violation and Divergent Timelines

"Missing One's Coach" AnonymousA Connecticut Yankee In King Arthur's Court by Mark TwainThe Time Machine by HG WellsLest Darkness Fall by L Sprague de CampBring The Jubilee by Ward MoorePast Times (a revised edition) by Poul AndersonThe Time Patrol by Poul AndersonThe Shield of Time by Poul AndersonJack Finney's time travel short stories collectedTime And Again by Jack FinneyFrom Time To Time by Jack Finney

Circular Causality

"The Chronic Argonauts" by HG Wells"A Stitch in Time" and "Chronoclasm" by John Wyndham"By His Bootstraps" and "-All You Zombies" by Robert HeinleinThe Door Into Summer by Robert HeinleinThe Technicolor Time Machine by Harry HarrisonBeyond The Barrier by Damon KnightThe Corridors Of Time by Poul AndersonThe Dancer From Atlantis by Poul AndersonThere Will Be Time by Poul AndersonBid Time Return by Richard MathesonThe Time Traveller's Wife by Audrey Niffenegger

The Curious Sub-Sub-Genre of Juvenile Historical Fantasy Time Travel Novels by English Women Writers

A Traveller In Time by Alison UttleyThe Moon Dial by Helen CresswellTom's Midnight Garden by Philippa PearceThe Story Of The Amulet by E Nesbitetc

Not to be included

The End Of Eternity by Isaac AsimovThe Legion Of Time by Jack WilliamsonThe Big Time by Fritz LeiberUp The Line by Robert SilverbergThe Man Who Folded Himself by David Gerrold11/22/63 by Stephen King

For discussion of some of these works, see here and here. An extra volume of the Archives would be an Editorial explanation of inclusions and exclusions. Apart from the circular causality paradox and the curious sub-sub-genre, I found it impossible to list the categories separately. Causality violation is possibly attempted in A Connecticut Yankee and subtly implied in The Time Machine before it occurs in Lest Darkness Fall, thus initiating a divergent timeline.

"Missing One's Coach" and the first five items in the "Circular Causality" list could form a collection.

Addendum, 5/6/12: This note belongs on my "Logic of Time Travel" blog where it can also be found. It was copied here by mistake but I decided to leave it here since works by Poul Anderson are such a major part of the proposed Time Travel Archives.